(i) Field of the Invention
The present invention relates to a hydrogenation catalyst for the production of hydrogen peroxide, a method for the production of the same, and a method for the production of hydrogen peroxide by the use of the hydrogenation catalyst. More specifically, the present invention relates to a catalyst which can be used in the hydrogenation reaction of anthraquinones in a hydrogen peroxide production process utilizing an anthraquinone method and which is excellent in activity, strength, life, selectivity and the like; a method for efficiently preparing this catalyst; and an industrially advantageous method for economically preparing hydrogen peroxide by the use of this catalyst.
(i) Description of the Prior Art
A main production process of hydrogen peroxide which has currently been practiced on an industrial scale is called an anthraquinone method in which anthraquinones are used as reaction media. In general, the anthraquinones are used by dissolving them in an appropriate organic solvent. This organic solvent may be a single organic solvent or a mixed organic solvent, but it is usually a mixture of two organic solvents. The solution obtained by dissolving the anthraquinones in the organic solvent is called "a working solution".
According to the anthraquinone method, the anthraquinones in the working solution are subjected to reduction (hereinafter referred to as "the hydrogenation") with hydrogen in the presence of a catalyst in a reduction step to produce corresponding anthrahydroquinones. Next, in an oxidation step, the anthrahydroquinones are oxidized with air or an oxygen-containing gas to convert the anthrahydroquinones into the anthraquinones again and to simultaneously produce hydrogen peroxide. Hydrogen peroxide produced in the working solution is usually extracted with water in an extraction step to be separated from the working solution. The working solution from which hydrogen peroxide has been separated is returned again to the reduction step, thereby forming a cyclic process. This cyclic process can produce hydrogen peroxide substantially from hydrogen and air, and hence it is an extremely efficient process. This cyclic process has already been used for the industrial production of hydrogen peroxide.
In this cyclic process, the reaction for the reduction of the anthraquinones is a very important step, and it is a large theme that this step is improved so as to be an excellent step from the viewpoints of operation and economy. This reduction reaction of the anthraquinones can be carried out by blowing hydrogen into a reactor in which the working solution containing the anthraquinones and the catalyst-are present. The blown hydrogen is first dissolved in the working solution, and this working solution is then stirred to disperse hydrogen and to simultaneously move hydrogen onto the surface of the catalyst. On the surface of the catalyst, hydrogen is reacted with the anthraquinones to produce the anthrahydroquinones.
In general, the reduction reaction of the anthraquinones is a very rapid catalytic reaction, and so this reaction is largely affected by a mass transfer rate of hydrogen. Thus, it is known that a reaction rate is limited by the transfer of a hydrogen gas to the working solution and additionally the transfer rate of the hydrogen gas onto the surface of the catalyst [e.g., Ind. Eng. Chem. Res., Vol. 33, p. 277-284 (1994)]. In order to raise the reaction rate, important are the development of a catalyst having a higher activity and the creation of such a reactor design that the mass transfer rate of hydrogen can be raised.
Furthermore, one of the main operation costs of the above-mentioned cyclic process is a catalyst cost. The activity of the catalyst deteriorates with the lapse of use time, and therefore, in order to maintain the desired hydrogenation reaction rate of the anthraquinones, the addition or the replacement of the catalyst is required. Accordingly, for the purpose of reducing the operation costs, it is necessary to use the catalyst having a high activity and a low deterioration rate.
As the catalyst for use in the hydrogenation of the anthraquinones, there are known a Raney nickel catalyst, a palladium black catalyst and a palladium catalyst supported on a carrier. The Raney nickel catalyst is highly active, but it has many drawbacks. For example, the Raney nickel catalyst noticeably deteriorates owing to a trace amount of hydrogen peroxide in the working solution, and it is an ignitable metal and so its handling is dangerous. In addition, its selectivity is low. The palladium black catalyst mentioned above is excellent in the activity and the selectivity, but its separation from the working solution is difficult. For the industrial production of hydrogen peroxide which is liable to decompose in the presence of palladium, the palladium black catalyst has the fatal disadvantage. On the other hand, with regard to the palladium catalyst supported on the carrier, its activity and selectivity are slightly inferior to those of the palladium black catalyst, but the separation of the palladium catalyst from the working solution is possible, and the palladium catalyst can be considered to be a catalyst suitable for the industrial production of hydrogen peroxide.
As the palladium catalyst supported on the carrier, there have been suggested catalysts supported on various carriers such as silica, alumina, silica-alumina, aluminosilicates, carbonates of alkaline earth metals and activated carbon, but all of these catalysts do not meet conditions such as a low cost, a high catalyst strength, a high activity and a high selectivity required as the industrial catalyst. Actually, a limited part alone of the above-mentioned catalysts can industrially be utilized.
The palladium catalyst supported on the alumina is one of a few catalysts which are industrially utilizable, and it has advantages such as a relatively high activity and an easy regeneration by calcination, but it also has a disadvantage that by-products are produced in large quantities during the hydrogenation of the anthraquinones and another disadvantage that the activity noticeably deteriorates owing to water in the working solution (U.S. Pat. No. 2,867,507). As an example of methods for preparing the palladium catalyst supported on the alumina, British Patent No. 718,306 has described a method which comprises impregnating a .gamma.-alumina carrier with a palladium salt, treating the carrier with a hydroxide of a metal or an aqueous carbonate solution, and then doing reduction with a hydrogen gas. Furthermore, Japanese Patent Publication No. 5120/1974 has suggested a method which comprises impregnating the alumina carrier with palladium, copper or silver, and another method which comprises further treating the alumina carrier at 150 to 650.degree. C. in a gas containing hydrogen after the impregnation.
The thus obtained catalyst can improve the selectivity of the hydrogenation of the anthraquinones to some extent. However, these preparation methods have some drawbacks. For example, a complicated operation is required, and since the heat treatment is carried out by the use of hydrogen gas, the operation is dangerous and hence these methods are not suitable for the mass production of the catalyst. In addition, the activity of the prepared catalyst unavoidably deteriorates owing to water in the working solution.
Furthermore, the palladium catalyst supported on silica is also one of a few catalysts which are industrially utilizable. With regard to the palladium catalyst supported on silica, Japanese Patent Publication No. 29588/1988 has suggested a catalyst to which at least one metal selected from the group consisting of zirconium, thorium, cerium, titanium and aluminum is added. In this catalyst, the deterioration due to water in the working solution does not occur in contrast to the palladium catalyst supported on the alumina, and the activity and life which are satisfactory to some extent can be obtained. Nevertheless, the improved activity and life cannot be considered to be sufficient.
If the preparation method of the palladium catalyst supported on a carrier other than silica is applied as the preparation method of the palladium catalyst supported on silica, a fatal problem takes place. That is to say, in this case, it is not considered that the adsorbability of a palladium compound on the carrier depends upon the physical and chemical characteristics of the carrier, and therefore palladium is apt to peel off from the prepared palladium catalyst supported on silica, so that the deterioration of the catalytic activity and the decomposition of hydrogen peroxide produced in the oxidation step tend to occur.
In U.S. Pat. No. 2,657,980, as an example of the preparation method of the palladium catalyst supported on .gamma.-alumina, there has been disclosed a preparation method which comprises supporting a palladium compound on .gamma.-alumina, and then reducing it with hydrogen or formaldehyde, and as a comparative example, a palladium catalyst supported on silica has been prepared in a similar manner. In the U.S. patent, it has been described that the catalyst has a low activity, and this fact implies that when silica is used as the carrier, the adsorption of PdCl.sub.4.sup.2- is insufficient, so that palladium is peeled off.
As preparation methods of the catalyst which can solve the problem of the palladium catalyst supported on silica, i.e., the peeling of palladium, there have been suggested some methods in which a treatment using a base is carried out prior to the supporting of the palladium compound on silica. In U.S. Pat. No. 2,940,833, sodium bicarbonate is used as the base, and in British Patent No. 776,991, an insoluble magnesium compound is used as the base.
However, it is not known that the palladium catalysts supported on silica prepared by these methods have been applied to the preparation of the hydrogenation catalyst for the industrial manufacture of hydrogen peroxide.
Furthermore, as a preparation method which can suppress the peeling of palladium, the above-mentioned Japanese Patent Publication No. 29588/1988 has suggested a method which comprises mixing a water-soluble palladium salt, silica and a water-soluble salt of at least one metal selected from the group consisting of zirconium, thorium, cerium, titanium and aluminum, and then regulating a the to support palladium and the metal in a state of a carbonate, an oxide or a carbonate on silica. In the publication, it has been described that in this method, the added metal compound functions as an deposition accelerator for accelerating the deposition of the palladium compound on the silica carrier.
However, this method has a drawback that a complicated operation is required. Particularly in order to surely carry out the support of palladium, the precise control of the regulation of the pH and the amount of the added metal is necessary.
As described above, the conventional preparation methods of the palladium catalyst supported on silica have been improved in the peeling of palladium to some extent, but they have some problems such as the poor strength of the prepared catalyst and the complicated operation. Accordingly, the conventional methods cannot be considered to be sufficient.
Furthermore, in the above-mentioned cyclic process for preparing hydrogen peroxide, the working solution is cyclically reused, and therefore alkyloxanthrones and alkyltetrahydroanthraquinones produced by the hydrogenation of the anthraquinones and other by-products which cannot produce hydrogen peroxide any more are slowly accumulated in the working solution, while the production of hydrogen peroxide is continued. The production of these by-products leads to not only the loss of fed hydrogen but also the loss of the expensive anthraquinones, which inconveniently increases the manufacturing cost of hydrogen peroxide. A part of these by-products can be returned to the original anthraquinones by a suitable treatment, but such a treatment results in the increase in the manufacturing cost of hydrogen peroxide. Therefore, the selectivity of the catalyst is an indispensable factor for the hydrogenation catalyst for the anthraquinones, and this factor is important on an equality with the strength, the activity and the life of the catalyst or is more important than them.
However, as described above, the conventional catalysts have been improved in its strength, activity and life to some extent, but its selectivity cannot be considered to be sufficient.
On the other hand, examples of a reactor which can be used in the present invention include a suspension bubble column comprising a tower container to which a gas and a liquid can be introduced in the presence of the catalyst, and a tank container equipped with a stirrer. The suspension bubble column has been used for many years because of a simple structure, and the absorption rate of the gas into the liquid depends largely on the area of an interface between the gas and the liquid [e.g., Fukuma et al., J. Chem. Eng. Japan, Vol. 20, p. 321 (1987)]. On the other hand, the mechanical stirring reactor can increase the mass transfer rate by mechanical stirring, and so this type of reactor has widely been used inclusive of an operation under pressure, except for a case where the amount of the gas is much larger as compared with that of the liquid. Also on the production of hydrogen peroxide, researches have been conducted, and there has been done the measurement of a hydrogen gas transfer rate in a case where the reduction reaction of the anthraquinones is carried out in the presence of the palladium catalyst in the mechanical stirring reactor [e.g., Ind. Eng. Chem. Res., Vol. 27, p. 780-784 (1988)].
However, when the mechanical stirring reactor or the suspension bubble column is used in the reduction step of the anthraquinone method, some problems occur. The catalyst for the reduction reaction collides against stirring blades and the wall of the reactor, so that the catalyst is pulverized to form a fine powder having a very small particle diameter. However, from the viewpoint of safety, a filter for preventing the fine powder from getting into a next oxidation step is required, and such a filter system is usually expensive. Furthermore, the fine powder formed by the above-mentioned pulverization causes the clogging of the filter on occasion. In addition, palladium tends to peel off from the carrier, so that a problem such as the deterioration of a catalytic activity comes up.
Usually, in the catalyst for the reduction of the anthraquinones, an expensive metal such as palladium is used, as described above, and therefore, when the above-mentioned problems occur, the system is economically disadvantageous.
When the rotational speed of the stirring blades is increased particularly in the mechanical stirring reactor in order to increase the reaction rate of the reduction, the above-mentioned problems are noticeable.
For the purpose of solving the problems regarding the reduction reaction of the anthraquinones, a fixed-bed type reactor can be employed as needed. In this case, it can be presumed that the pulverization and the wear of the above-mentioned catalyst for the reduction decrease, and the cost is lower than in the filter system. However, when the fixed-bed type reactor is employed for the reduction of the anthraquinones, the following problems are present. A hydrogen gas transfer rate, i.e., a dissolving rate of the hydrogen gas into the working solution, and additionally the transfer rate of the hydrogen gas onto the surface of the catalyst are low, and so a reduction rate cannot be increased; and pores in the catalyst cannot be utilized as effective reaction sites. Therefore, in the case that the fixed-bed type reactor is employed for the reduction of the anthraquinones, a technique for increasing the mass transfer rate is necessary. In U.S. Pat. No. 2,837,411, a device for previously bringing hydrogen to be introduced into the reactor into contact with the working solution is installed, and in U.S. Pat. No. 4,428,922, a technique has been suggested in which the working solution is mixed with hydrogen by a static mixer prior to introducing them into the reactor. In both the techniques, however, the amount of the working solution to be circulated swells and the consumption of hydrogen increases, and for these reasons, this fixed-bed type reactor is not always economically advantageous. In addition, U.S. Pat. No. 4,552,748 has suggested a reduction reaction device having a honeycomb structure, but in this case, the removal of reaction heat is tardy for a structural reason, so that a temperature in the vicinity of the center of the honeycomb rises and the allover uniformity of a reaction temperature is inconveniently lost. In addition, it is difficult to uniformly disperse the hydrogen gas in the working solution, so that the uniformity of the reduction reaction is also lost.
In EP Patent No. 0384905, the hydrogen gas and the working solution are introduced into the fixed-bed type reactor through its upper portion, and the introduction speed of the working solution is set so as to be lower than the speed of the working solution which downward flows through the fixed bed by its weight, whereby a higher reaction rate than in the conventional fixed-bed type reactor can be obtained. Also in this case, however, the production rate of hydrogen peroxide per weight of the catalyst is lower as compared with the mechanical stirring reactor and the suspension bubble column.
In this connection, as a technique regarding the hydrogenation catalyst for use in the hydrogen peroxide manufacturing process utilizing the anthraquinone method, WO 96/18574 is present in addition to the above-mentioned techniques, and as a production method of hydrogen peroxide, U.S. Pat. No. 5,399,333.